Honey bees have a negligible amount of phosphomannoseisomerase, together with a high content of a hexokinase which phosphorylates mannose more efficiently than fructose or glucose. Competition at the phosphorylation level plus accumulation of mannose-6-phosphate can fully account for the toxicity of mannose in honey bees.

Nitrate uptake has been studied in nitrogen-deficient cells of the marine diatom Skektowema costatum. When Nitrogen appears to be the major nutrient limiting primary production in the oceans of the world (18), as well as in certain freshwater systems. Nitrogen is found in seawater as dissolved N2 (which cannot be fixed by most marine algae) and as inorganic ions: nitrate, nitrite, and ammonium. These ions are present in concentrations ranging from 0.01 to 50 pM for nitrate, from 0.01 to 5 Am for nitrite, and from 0.1 to 5 pM for ammonium. The concentration of organic forms is usually below 10 pm (15). When all three inorganic ions are present, ammonium is preferentially utilized (4). Nitrate and nitrite have to be reduced to ammonium by means of an energy-dependent enzyme system (nitrate reductase and nitrite reductase) prior to their assimilation by the cell (15).As nitrate is the most abundant form of N in seawater, marine phytoplankton, mostly composed of diatoms and dinoflagellates, utilizes nitrate as the main N source. Its uptake and reduction constitute major functions in these organisms, since N has been identified as the limiting factor controlling their growth. The relevance of the study of these processes arises from the fact that phytoplankton organisms are the major primary producers of the sea.The purpose of this paper is to characterize the kinetics of nitrate uptake in a phytoplankton organism of particular ecological significance because of its wide distribution, such as the diatom Skeletonema costatum. MATERIALS AND METHODSCulture Media and Conditions. Culture media were prepared according to Guillard and Ryther (8), with the modifications

(20), containing 100 .lM Na2WO4.Nitrate uptake assays, nitrate and nitrite determinations were performed as described in the previous paper (20).Nitrate is the most common form of N available to marine phytoplankton. Phytoplankton can take up nitrate at extremely low (<0.1 UM) concentrations and up a negative gradient, suggesting an active transport mechanism. Ammonium appears to be the primary inorganic N form entering the biosynthetic pathways. Nitrate as well as nitrite must be reduced to ammonium, by means of nitrate reductase and nitrite reductase, prior to assimilation. Nitrate induces the development of nitrate reductase and nitrite reductase activities in a wide range of plant species (1, 5) as well as in Skeletonema costatum (19). Ammonium has been shown to inhibit the development of these enzyme activities (16)(17)(18)(19). Ammonium does not only inhibit the de novo synthesis of nitrate reductase, but also inactivates the preexisting enzyme (12,19), which leads to a rapid decrease in nitrate reduction. Nitrate reductase activity seems to be more easily inhibited by ammonium than induced by nitrate, so that very low, if any, enzyme levels are detected in cells growing simultaneously in the presence of nitrate and ammonium. Under these conditions, it was observed that the cells incorporate first ammonium, and start taking up nitrate only when all of the available ammonium has been exhausted (24).Two steps can be clearly distinguished in nitrate assimilation: the transfer of the ion from outside the cell into the cytoplasm and its further reduction. Both processes appear to be intimately linked, but can be separated easily by means of tungstate. Tungstate interacts with nitrate reductase in order to give an inactive form of the enzyme (10, 13) so that nitrate uptake can be studied separately.The present paper deals with the regulatory effect of ammonium, nitrite, and amino acids on the isolated nitrate uptake system, once nitrate reductase activity has been inhibited by tungstate. '

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